British citizens who travel abroad to fight alongside the militant group Islamic State of Iraq and Greater Syria (ISIS) will be prevented from returning to the U.K. for two years and only allowed to re-enter if they consent to face trial, home detention, police surveillance or attend a de-radicalization course, the Guardian reports.

The plans, revealed by Prime Minister David Cameron in a speech to the Australian parliament in Canberra, follow a pledge Cameron made in September to increase counterterrorism efforts after the U.K. raised its terror threat level to “severe.”

Security services believe up to 500 Britons have travelled to Syria, many of whom are aged between 16 and 21.

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